The invention relates to retroreflective particles for providing increased nighttime visibility of highway markings, signs and other surfaces where retroreflectivity is desired. The invention further relates to processes for the preparation of such particles and for their use.
The invention, while of general application, is particularly effective in adding retroreflectivity to roadway markings such that nighttime visibility of such markings is greatly improved. It is known that glass particles greatly increase the visibility of painted markings on roadway surfaces and other painted surfaces when the glass particles are evenly dispersed into the painted surface. However, evenly dispersing glass particles into a painted surface can be difficult unless the particles are free flowing. Because the particles used are typically so small as to have a powdery appearance, small amounts of moisture can cause the particles to agglomerate and lose important free-flowing properties. Even high relative humidity or condensation on the particle surfaces caused by temperature changes can provide sufficient moisture to cause agglomeration and to interfere with flow properties.
Agglomeration can be significantly reduced by applying hydrophobic coatings to the surface of the glass particles. These coatings reduce surface energy on the glass and keep water from binding particles to one another. A problem with hydrophobically coated particles, and to a significant degree with uncoated particles, is an inability of the particles to firmly bind the paints or other materials into which they are placed. Thus increased visibility imparted by hydrophobically coated glass particles can be short-lived due to a steady loss of the glass particles as they are loosened and removed by friction, weather changes and other physical factors.
The loss of glass particles can be slowed by coating the particles with thin layers of certain coupling agents selected for their ability to provide positive adhesion between the particles and surrounding materials. Known coupling agents however reduce the retroreflectivity of glass particles by causing a process known as "wicking" by which paints are drawn onto the glass surface and coat too much of the surface area for sufficient retroreflectivity to be achieved. Although normal road abrasion wears away some of the paint which results from wicking, retroreflectivity never becomes ideal, and in any event is diminished for an undesirably long period of time after the glass is placed. Additionally, known coupling agents are not hydrophobic and particles coated therewith are subject to moisture induced agglomeration with all of the attendant difficulties described above.
It is accordingly an object of the present invention to provide materials capable of producing optimal retroreflectivity which are resistant to agglomeration even under moist conditions, and which will additionally exhibit improved adhesion to surfaces to which they are applied.
It is a further object to provide retroreflective materials which are functional and effective shortly after application.
Still another object is to provide retroreflective materials which are long-lasting, durable, economical and thoroughly reliable in use.